Some common types of high intensity discharge lamps used for general lighting comprise a quartz or fused silica arc tube enclosed within a glass outer jacket fitted with a screw base at one end. In high pressure mercury vapor lamps the arc tube contains a filling of mercury, whereas in high pressure metal halide lamps, the arc tube contains a filling of mercury and metal halides. In both kinds, the arc tube transmits ultraviolet radiation which is harmlessly absorbed by the glass outer envelope, or even gainfully absorbed by a phosphor coating on the other envelope.
Most commonly the outer envelope remains intact to the end, the lamp life is ended by other factors. However it does happen occasionally that the outer envelope or glass jacket is shattered and the arc tube remains intact so that the lamp continues to operate. In this mode of operation, the ultraviolet radiation from the arc tube is not intercepted and may create a safety hazard.
A solution proposed to this problem by copending application Ser. No. 669,289 of Eugene K. Corbley, filed of even date herewith, now U.S. Pat. No. 4,039,893, titled "Discharge Lamp Having Disconnect Effective Upon Jacket Failure" and assigned like this application, utilizes a mechanical disconnect located in the outer envelope. The disconnect comprises conductors which are part of the arc tube supporting frame and which are held in engagement by the outer envelope. Upon fracture of the outer envelope, spring pressure forces the conductors apart whereupon the circuit is opened and the arc tube is disabled. While this arrangement is effective and achieves its intended purpose, a reliable, simpler and more economical solution is desired.